Fallopia japonica var. compacta. As the name suggests, is the dwarf variety of Japanese Knotweed, and compared with its larger variety it is very well-behaved. It is rarely naturalised in Europe and when it is, it does not occupy very extensive areas. From our point of view its main importance is that since both sexes occur it has been able to get involved in hybridisation with F. sachalinensis.
Fallopia sachalinensis (Giant Knotweed). This is a close relative of F. japonica, but has a more northerly distribution, being restricted to Northern Japan and Sakhalin Island. It is a much taller plant with much larger leaves, and although nothing like as common as Japanese knotweed, it does occur as both male and female plants and has hybridised with both it and its dwarf variety to produce the hybrid F. x bohemica.
Fallopia x bohemica. This is the hybrid between Japanese Knotweed and Giant Knotweed. It is now known from nearly 200 locations in Britain alone, and is also common in a number of continental European countries. If this plant were to pollinate giant knotweed, then fertile F x bohemica would be produced able to cross pollinate the female japanese knotweed and allowing it to reproduce by seed as well!
In order to further understand the fertility of the knotweeds it is necessary to look at the chromosome numbers of these plants, since they are found at different chromosome levels, and these chromosome levels determine the fertility of the plants.
Origins of the 2n=44 and 2n=66 F. x bohemica plants
The 2n=44 plants are known as tetraploids as they contain 4 sets of chromosomes, the 2n=66 plants have 6 sets of chromosomes and are known as hexaploids. The tetraploid plants are perfectly fertile and can interbreed with either of their parents, but since the tetraploid hybrids are very rare this is not yet known to occur anywhere. Conversely, the hexaploid plants are reasonably common but only partly fertile, and any pollen produced usually contains between 30 and 66 chromosomes. If a pollen grain with 66 chromosomes were to pollinate a Fallopia sachalinensis flower a fertile octoploid F. x bohemica would be produced, such plants would be able to cross-pollinate our all female Japanese Knotweeds and potentially be a replacement for the absent male Japanese Knotweed, allowing Japanese Knotweed to reproduce by seed again!
The figure shows the known extent of hybridisation, many of these hybrids are known only from seed collected from Knotweed plants, that is we know that these hybrids are being produced, but for some reason are not generally becoming established. They can be distinguished on the basis of their morphology and sex-expression. Any large Knotweed plant that is male-fertile, and that is not F. sachalinenisis is likely to be the hybrid F.x bohemica. This can be confirmed by an examination of the underside of the larger leaves, where a good hand-lens or low power microscope will reveal numerous short pointed hairs. It should be remembered that even female plants of Japanese knotweeds have stamens. But these are minute, contain no pollen and are retained in the flower, often appressed to ovary. Fertile anthers are only found on male plants and are much larger, borne on long filaments which protrude from the flowers and clearly contain pollen. The other character is that of leaf shape, the principle shapes are shown in Fig X.
Leicester University keeps records of all known Knotweed hybrids and would greatly appreciate reports of suspected hybrids, along with a 6 figure grid reference and a dried leaf and if possible flowers from the plant. A single large leaf and small piece of inflorescence can be easily dried in a telephone directory in a couple of days.
Please send any
specimens and details to Dr John Bailey, Biology Department, University of Leicester,
LEICESTER LE1 7RH
http://www.le.ac.uk/biology/staff/jpb/main.htm
The most important feature of all this is that it has managed to invade the whole of the British Isles by vegetative reproduction alone - seed has played absolutely no part in the invasion process. To say that it cannot reproduce itself by seed is rather a fine point. Whilst this is of course true in the strictest sense, in that it cannot produce seed of the same species due to the absence of male plants of the same species and the same chromosome number. It can nonetheless reproduce by hybridisation, and since there are thousands of hectares of densely flowered female plants throughout Britain, one may be assured that absolutely anything that is capable of pollinating it - will have pollinated it! Even though male plants of both F. japonica var. compacta and F. sachalinensis can both pollinate Japanese Knotweed, these plants are not very common, and sites where these male plants occur close to a Japanese Knotweed plant are even rarer, and so are rarely pollinators. Studies of the seed produced by Japanese Knotweed have shown that it is most commonly pollinated by the common garden plant Russian Vine or 'Mile a minute Vine' (Fallopia baldschuanica). This was a most unexpected result, and its discovery provided additional evidence in support of the amalgamation of the genera Fallopia and Reynoutria. Thankfully such hybrids do not combine the worst traits of each parent and are actually less vigorous than either parent, probably because each has a different overwintering strategy - underground rhizomes in Japanese Knotweed - overground woody stems in Russian Vine.
It is a curious fact, that although much viable seed is produced by Japanese Knotweed, it almost never germinates in the wild. However, in the collection at Leicester in the spring of 1986 seedlings were observed growing between cracks in the paving stones and in the gutters. It is probably no coincidence that the preceeding winter was one of the coldest in many years. This has led us to suspect that our winters may actually be too warm and wet for these relatively thin-walled seeds and that they probably rot in our winters.